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Delayed Time-base (see Fig. 8.1 1 )
The DELAYED TIME-BASE GENERATOR itself generates a time-linear sawtooth in the same way as
described for the main time-base.
Transistor V229 is the constant-current source, with its base fed from a fixed d.c. voltage that is derived via
the continuous TIME/DIV control R9. The base voltage of V229 i s only changed if R9 is moved out of the
CAL position. By means of the TIME/DIV switch S13, eight different emitter resistors can be selected for
current source V229. Depending on the position o f S13, only one timing capacitor C219 (fast sweep speeds)
or two timing capacitors C219/C218 (slow sweep speeds) are switched into the circuit. Capacitor C218 is
switched into the circuit by means of transistor V231.
This transistor functions in the 'reversed' mode (collector and emitter are reversed) during the charge of the
timing capacitor and in the 'normal' way during the discharge.
Switching of V231 is controlled by TIME/DIV switch S13. The table below indicates the capacitors and
adjustment potentiometers that are in circuit as a function of the position of S13.
...
5 ps/DIV
...
I ms/DIV
a) COMPA RA TOR
This part of the circuit consists of transistors V259, V248 and current source V268 and it compares the MTB
sawtooth voltage (applied t o the base of V259) with an adjustable d.c. voltage from DELAY TIME potentio-
meter R3 (applied t o the base of V248 via Darlington pair V246, V247).
A t the moment that the instantaneous d.c. value o f the MTB sawtooth exceeds the voltage on the base of
NAND-gate input D203-4 becomes H.
The comparator has a current source V268, which is switched on if the lower end of R358 i s connected t o
earth via S13. With the DTB TIME/DIV switch S'13 in the OFF position, R358 i s floating and the current
source i s switched off. As a result, the comparator i s inoperative and the DTB cannot be started.
The voltage
both ends of the DELAY TIME control R3 i s adjustable by presets R262 and R268.
a t
Before the start of the DTB, the position of t h e SR flip-flop D204 (1,2,3) and D204 (4,5,6) i s as follows:
Output D204-1 - L
Output D204-4 - H
Input D203-4 becomes H if the comparator switches over. Input D203-5 is H (during the MTB sweep).
Consequently, output D203-6 becomes L together with input D204-12.
Input D204-11 is L, thus output D204-13 becomes H and is applied t o the reset input, pin 1 of flip-flop D207.
There are now two start possibilities for the DTB:
-
Pushbutton MTB of S22 i s depressed: the 'START' mode is selected and the DTB starts directly after the
selected DELAY TIME.
I n this mode, the
set
signal on onput pin 1 of D207 appears on the output pin 6. I f pin 1 goes
switching transistor V228 turns off and the DTB starts.
-
Pushbutton A or B of S22 i s depressed: the TRIGGERED mode is selected and the DTB starts after the
pre-selected time delay, only upon receipt of a trigger pulse.
In this mode, the set input D207-4 is H, and 0207 now functions as a normal flip-flop.
1
After pin
has
This clock-pulse occurs if V226, V212 (the CURRENT-VOLTAGE CONVERTER for the selected trigger
source) and emitter-follower V225 make inverter inputs D203-12/13 logic L. As a result, the clock input
H
D207-3 goes
and the flip-flop switches over: output pin 6 goes
Timing capacitors
in circuit
C219
(see
timing diagram Fig.
input, pin 4 of flip-flop D207 is connected t o earth,
Adjustment point
R349
R351
so
H,
i s
L
and switch V228 now goes non-conduc-
the inverted form of the
then output pin 6 goes
L,
the
2-25
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